Good gods.....
I tried all size and styles of tub, oversized heat tape (12"), narrow heat tape (3"), different substrates, few holes, many holes, bottom heat, back heat, side heat, back and side, bottom and back.... all to no avail.
The cool side never gained much heat from the warm, maybe a few degrees but not enough. I always ended up with a warm side at 90 F and a cool at 73, at best 74F.
A rack system would be the greatest freaking thing! Can you please give details on your set up?